1. Field of the Invention
The present invention relates to semiconductor laser structures.
2. Description of Related Art
In order to fabricate semiconductor lasers with superior performance characteristics, conventional designs require complex processing of the multi-layer laser material at the wafer level. Some of these complexities can be avoided by using a design in which the cladding layer closest to the wafer surface (usually the p-type cladding layer) is made much thinner than usual.
In these thin p-clad designs as well as in standard thick p-clad designs, the top layer of the laser material (cap layer) is usually heavily p-doped for ohmic contact purposes. In the thin p-clad designs, conventional wisdom dictates that the cap layer must be made very thin in order to avoid large optical loss.
Another critical step in the manufacture of semiconductor diode lasers is the growth of epitaxial material on top of the processed material. The probability for material loss during this "regrowth" process is high, leading to increased cost for the lasers fabricated from such material. The regrowth step is avoided by using a thin p-clad design as discussed above. In these thin p-clad lasers, the lasing mode interacts with the surface of the laser, allowing one to tailor the characteristics of the lasing mode (and, hence, the laser beam) with surface processing.
It has been demonstrated previously that diffraction gratings fabricated in the surface of thin p-clad lasers can force the laser to operate at a single frequency with high output power. It has been difficult, however, to make such lasers produce a spatially coherent beam, limiting their utility to "low brightness" applications.
In controlling the lateral beam characteristics of semiconductor lasers, it is desirable to utilize a design which allows the fabrication of refractive index variations in the material in a region close to the quantum well (QW) active region, in a precise reproducible way. In principle, this can be achieved by etching precise structural variations into thin p-clad laser material since regrowth over the structural variations is not required [Macomber et al, Proc. SPIE, Vol. 3001, pages 42-54 (February 1997)]. In this case, it is important that the reflectivity of the metal contact-semiconductor interface be high at the lasing wavelength if low threshold current density operation is to be achieved [Wu et al, IEEE Photonics Tech. Lett., Vol. 6, pages 1427-1429 (December 1994)]. This article is hereby expressly incorporated by reference in this application.
The first work on diode lasers using a thin p-clad design was conducted over 20 years ago [Zory et al, IEEE J. Quantum Electron., Vol. QE-11, "Grating-coupled double-heterostructure AlGaAs diode lasers," pages 451-457 (1975); and Walpole et al, Appl. Phys. Lett., Vol. 30, "CW operation of distributed feedback Pb.sub.1-x Sn.sub.x Te lasers," pages 524-526 (1977)]. The work on such lasers was motivated by the fact that corrugated diffraction gratings could be incorporated into the laser material without any material regrowth over the corrugated surface. The performance of such devices was limited because of the layer uniformity problem inherent in the liquid phase epitaxy process used in growing the laser material. With the development of the MOCVD and MBE crystal growth techniques in the 1980's, work on corrugated thin p-clad lasers was revived, with substantial improvements in performance being achieved compared to the earlier work [Macomber et al, Appl. Phys. Lett., Vol. 51, "Surface emitting distributed feedback semiconductor lasers," pages 472-474 (1987); Shani et al, IEEE J. Quantum Electron., Vol. QE-24, "Metal clad Pb.sub.1-x Sn.sub.x Se/Pb.sub.1-xy Eu.sub.y Sn.sub.x Se distributed feedback lasers," pages 2135-2137 (1988); Rast et al, IEEE Photonics Tech. Lett., Vol. 7, "Gain-coupled strained layer MQW-DFB lasers with an essentially simplified fabrication process for=1.55 .mu.m," pages 830-832 (1995); and Smith et al, Proceedings of IEEE/LEOS Annual Meeting, San Francisco, Calif., "Wavelength tunable asymmetric cladding ridge waveguide distributed bragg reflector lasers," pages 294-295, (1995)]. Recent work [Macomber et al, Proc. SPIE, supra] indicates that surface emitting 2D laser arrays based on the corrugated thin p-clad laser concept are improving rapidly and show great promise for use in both commercial and defense-related applications.
It is an important object of the present invention to provide a semiconductor laser structure that is relatively simply to fabricate and which operates as a high power, high brightness, coherent semiconductor laser.